1894.] TADPOLE OP XENOPUS LjEVIS. 107 



directed processes from the first and second branchial arches ; they 

 are somewhat conical in shape, with rounded or very slightly 

 notched hinder borders." This description applies in the main to 

 tadpoles of Xenopus of May 31st. The opercular fold is then only 

 commencing to grow, and processes from the three first branchial 

 arches just project beyond the line of the body. In the lax tissue 

 lying in the interior of these processes is a capillary vessel derived 

 from the vascular arch. The processes, however, are hardly 

 conical in form ; they have a long base of attachment, and are 

 indeed rather to be described as lamellae than processes. 



Pronephros. — I carefully investigated the pronephros, but with 

 entirely negative results so far as the discovery of anything of 

 novelty is concerned. It is precisely like that of JRana, and opens 

 into the body-cavity by three funnels opposite to its glomerulus. 



Vascular System. — Messrs. Marshall and Bles have described with 

 such minuteness the development of the heart and arterial system 

 in Rana temporaria that a comparison with the corresponding 

 stages of Xenopus becomes easy. It is very remarkable, as they 

 point out, that the condition of the vascular arches should differ 

 so much from that of the closely-allied Rana esculenta. In the 

 latter, according to Maurer (quoted by Messrs. Marshall and 

 Bles), the afferent and efferent branchial vessels are continuous 

 with each other, forming complete arches. In one specimen of 

 Rana temporaria the same continuity was noted, but as a rule the 

 communication between afferent and efferent sections of the aortic 

 arches was indirect through the branchial capillaries. In view of 

 this difference between two species of one genus, the fact that 

 Xenopus agrees with Rana esculenta is of less interest. In Xenopus 

 it is quite easy to trace the four aortic arches from the heart to 

 the dorsal aortae. 



The truncus arteriosus first divides into two branches (on each 

 side) ; the posterior of these again divides into two, and a little 

 later the vessel which is now the hindermost itself divides into 

 two trunks ; thus the four afferent bronchial vessels arise. Messrs. 

 Marshall and Bles figure (he. cit. pi. xiv. fig. 6, A H) a short 

 diverticulum of the truncus arteriosus lying in front of the fully- 

 developed first branchial arch in tadpoles of 5 milium ; this they 

 consider to be referable to the byoid arch. It disappears soon. I 

 find an entirely similar diverticulum of the first arch in Xenopus 

 in a tadpole of 7 millim. ; it was present on both sides of the 

 body. In tadpoles of June 2nd there were only three vascular 

 arches. The fourth arch, arising from the third, went straight 

 to the lung. 



EXPLANATION OF PLATE XIII. 



Fig. 1. Tadpole of Xenopus Icevis of June 5th. 



Figs. 2, 3. Dorsal and ventral views of an older tadpole. 



Figs. 4, 5. Lateral and dorsal views of a full-sized tadpole. 



